Democratic candidate Jon Ossoff hoping to capture congressional district from Republicans for the first time since 1979.

A young and inexperienced Democratic candidate could win a congressional race in the Atlanta suburbs where Democrats hope to defy President Donald Trump by capturing a seat held by Republicans since 1979.

As results trickled in on Tuesday night, Jon Ossoff, 30, narrowly led a field of 18 candidates vying for a US House of Representatives seat that was vacated when Trump named Tom Price as his health secretary.

If Ossoff fails to win an outright majority of the vote in Georgia's 6th District, he will enter a June 20 runoff with one of the 11 Republicans in the race.

With 113 of 210 precincts counted, Ossoff, a documentary filmmaker and former congressional aide, held 50.3 percent of the vote.

With the slogan "Make Trump Furious," Ossoff aimed to galvanise opposition to a president struggling with an approval rating that has not topped 50 percent since he took office on January 20, according to Reuters/Ipsos polling.

Ossoff has drawn volunteers and donors from out of state who see the race as a way to strike a blow against Trump. He raised a stunning $8.3m in the first three months of the year.

An Ossoff win would not tip the balance of power in Washington but could weaken the already shaky hold Trump has on his fellow Republicans by encouraging lawmakers to distance themselves from him.

The district, which encompasses a stretch of well-heeled suburbs north of Atlanta, has elected Republicans to the House since the late 1970s, but Trump carried it by only 1 percentage point in the November presidential election.

Republicans in the race are split among Trump supporters and candidates trying to hold the president at arm's length.

'Fresh face in Congress'

Greg Blustein, a political reporter of the Atlanta Journal Constitution, said Ossoff's pledge to stand up to Trump resonates with voters.

"Voters here in the district also like his spirit, the fact that he's young and the fact that he would be a fresh face in Congress," he said.

Trump criticised Ossoff on Twitter and said in a robocall that the Democrat would "raise your taxes, destroy your healthcare, and flood our country with illegal immigrants".

Democrat Jon Ossoff would be a disaster in Congress. VERY weak on crime and illegal immigration, bad for jobs and wants higher taxes. Say NO

Ossoff said on CNN that Trump was misinformed about his positions and that he was focused on issues affecting the region, not Washington.

Republicans have responded with millions of dollars of spending of their own, casting Ossoff as an inexperienced outsider who does not live in the area he hopes to represent.

Ossoff, who grew up in the district, says he will move back if he wins.

The Republican Party avoided embarrassment last week when it narrowly held a conservative Kansas seat vacated when Trump tapped Republican Representative Mike Pompeo to head the Central Intelligence Agency.